The present invention relates to the field of delivery of cryogenic liquids, and comprises a unique system for delivering such liquids at controlled low pressures.
A cryogenic liquid, such as liquid nitrogen, is typically stored at high pressures, of the order of 25 psi, or even 100 psi. Many industrial processes require liquid nitrogen to be delivered at very low and precisely-controlled pressures, of the order of 1-5 psi.
One process requiring a cryogenic liquid at a precisely controlled pressure is the filling of aluminum cans. Because of its relatively low cost, aluminum is desirable as a packaging material for foods and beverages. But aluminum, being a soft metal, deforms easily when subjected to pressure. Unless the aluminum can is filled with a carbonated liquid, which provides sufficient internal pressure to resist external forces, the can is not easily stacked. When stacking is attempted, the cans near the bottom of the stack bend under the overall weight of the stack.
To increase the capacity of an aluminum can to withstand higher pressures, a small but controlled amount of liquid cryogen is injected into the can before it is sealed. The liquid cryogen vaporizes inside the can. Because the volume of the vapor is of the order of about 700 times the volume of the liquid cryogen, the pressure in the can is substantially increased, and the can is now able to resist substantial external pressures. The gas resulting from the injected liquid cryogen thus serves the same function as the gas in a canned carbonated beverage.
The machinery used to fill aluminum cans with liquid cryogens is designed to operate at a certain rate of mass flow. The mass flow rate of liquid cryogen depends directly on the pressure in the delivery line. It is therefore important not only to deliver the cryogen in a single phase, i.e. liquid, but also to control precisely the pressure in the delivery line. Deviation of the pressure in the line, from the desired value, causes the wrong amount of liquid cryogen to be injected into the can, and can result in an unacceptable internal can pressure.
Various means of delivering cryogenic liquids have been described in the prior art. For example, U.S. Pat. No. 3,696,627 shows a system for transferring a cryogenic liquid at constant temperature, pressure, and flow rate. The patent discloses a structure comprising a transfer tube which is jacketed within a second tube, substantially along its entire length. U.S. Pat. No. 3,827,246 discloses another device for controlling pressure of cryogenic liquids, the device employing means for recirculating part of the output fluid into the container. The latter invention, however, maintains the cryogenic fluid at very high, and not low pressures.
U.S. Pat. No. 4,336,689 discloses a method of delivering cryogenic liquids at a constant flow rate. The means of regulation is primarily a throttling valve, which reduces the rate of flow from the high pressure source. U.S. Pat. No. 3,440,829 discloses another pressure regulating system which increases the gas head pressure as a means of maintaining the pressure of the output liquid at a desired minimum level.
The present invention provides a system for delivering liquid cryogens at a controlled low pressure, the system being comparatively simple of construction, and inexpensive to operate. The system regulates the pressure of the liquid in part by regulating the pressure of the gas located above the liquid.